YELLOWFIN TUNA (THUNNUS ALBACARES) IN THE MALDIVES M. Shiham Adam and R. Charles Anderson Marine Research Section Ministry of Fisheries and Agriculture Malé Republic of Maldives
Since then they have declined, reflecting the general INTRODUCTION decline of the whole troll fishery as a result of changing socioeconomic conditions within the Maldives. Yellowfin (Thunnus albacares) is the second most important species of fish caught in the Maldives, after During the 1970-1994 period total yellowfin catches have skipjack tuna (Katsuwonus pelamis). Yellowfin catches increased substantially, but somewhat erratically (Figure have increased dramatically in recent years. In 1994 the 1). Catches from 1973 to 1982 averaged about 4400 t/yr, total Maldives catch of yellowfin was over 13,000 t (Anon, and from 1983 to 1990 about 6200 t/yr, but since 1990 they 1995), which was 12.6% of the total national fish landings. have more than doubled, to a record of 13,126 t in 1994. This is an increase of roughly 2000 t/yr during the 1990- The Maldivian yellowfin fishery is essentially a live-bait 1994 period. Explanations for this dramatic increase pole-and-line fishery. Catches by traditional (but now include: mechanized) pole-and-line vessels account for over 95% of 1. An increase in crude fishing effort. The number of the total catch. The yellowfin caught are mainly surface- active pole-and-line fishing vessels increased about swimming juveniles within the size range of 30-60cm FL. 23%, from about 1100 in 1990 to over 1400 in 1994. A Yellowfin are also caught regularly, but in smaller more useful index of fishing effort is the number of quantities, by handlining and trolling. These methods days fished by mechanised pole-and-line vessels, which generally catch large-size yellowfin of more than 70cm FL. increased 13%, from about 198,000 days in 1990 to In addition, longliners operating in the waters around the over 223,000 days in 1994 (see Anderson, Hafiz and Maldives take deep-swimming adult fish. Adam (1995) for details of fishing effort statistics). The Maldives “yellowfin” catch includes a small number of 2. An increase in fishing power of the pole-and-line bigeye tuna (Thunnus obesus). No separate statistics are vessels. In recent years boat owners have tended to kept for this species, but preliminary studies suggest that build bigger vessels and install larger engines, with the bigeye tuna may account about up to 5% of the total aim of increasing fishing power and attracting better yellowfin catch (Anderson and Hafiz, 1991; Anderson, crews. The introduction of mechanical pumps for 1995). spraying water during pole-and-line fishing, more frequent use of radios between vessels, greater use of This report summarises current knowledge of the biology binoculars for spotting seabirds, and an increase in the of yellowfin tuna in the Maldives, and presents new number of FADs have all increased the effective fishing information about the Maldivian yellowfin fishery. power of the vessels (Hafiz and Anderson, 1994). These increases are however difficult, if not impossible too quantify. CATCH TRENDS
Recorded catches of yellowfin tuna for the 1970- 1994 period are given in Table 1 and Figure 1. Figure 1. Maldives yellowfin catch by vessel type, 1970-1994. Figure 2 illustrates the relative contributions of the 14000 Sailing P/L Mechanized P/L Trolling main vessel types to annual catches. Pole and line 12000 is clearly the most important fishing method for yellowfin tuna in the Maldives. The pole-and-line 10000 fleet is a traditional one, but mechanization (which 8000 tch (t) started in 1974-75) effectively transformed the a 6000 C entire fleet from sail to engine power in less than 4000 ten years. The contribution of the trolling fleet to the total yellowfin catch is relatively small. 2000
Trolling catches peaked at about 590 t/yr (i.e., 0 about 14% of the total yellowfin catch) in 1979-80. 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994
1 Table 1. Catches and catch per unit effort Table 2. Maldivian yellowfin tuna catches by vessel type (1970- (CPUE) of yellowfin tuna for mechanized 1994). pole and line vessels, 1979-1994. Year Sailing P/L Mech. P/l Total P/L Trolling Grand total Year Catch Effort CPUE vessels vessels vessels 1970 1,799 - 1,799 190 1,989 (t) (days) (kg/day) 1971 1,081 - 1,081 146 1,227 1979 4,289 79,904 54 1972 1,940 - 1,940 136 2,076 1980 4,229 83,134 51 1973 5,234 - 5,234 241 5,475 1981 5,284 83,731 63 1974 3,868 - 3,868 260 4,128 1982 4,004 97,085 41 1975 3,348 164 3,512 262 3,774 1983 6,241 117,172 53 1976 3,569 912 4,481 410 4,891 1984 7,124 153,460 46 1977 2,530 1,593 4,123 350 4,473 1985 6,066 162,430 37 1978 1,324 1,890 3,214 370 3,584 1986 5,321 161,910 33 1979 733 2,959 3,692 597 4,289 1987 6,668 158,785 42 1980 471 3,176 3,647 582 4,229 1988 6,535 184,353 35 1981 273 4,467 4,740 544 5,284 1989 6,082 183,944 33 1982 167 3,603 3,770 234 4,004 1990 5,280 193,045 27 1983 112 5,872 5,984 257 6,241 1991 7,711 198,320 39 1984 76 6,818 6,894 230 7,124 1992 8,697 204,808 42 1985 82 5,715 5,797 269 6,066 1993 10,110 222,548 45 1986 22 5,178 5,200 121 5,321 1994 13,126 223,095 59 1987 9 6,522 6,531 137 6,668 Source: Ministry of Fisheries & Agriculture /EPCS. 1988 12 6,366 6,378 157 6,535 1989 6 5,972 5,978 104 6,082 1. A change in catchability and/or abundance of the 1990 5 5,225 5,230 50 5,280 fish. The abundance and/or catchability of yellowfin 1991 5 7,649 7,654 57 7,711 and other tuna species is known to be affected by 1992 11 8,628 8,639 58 8,697 changes in oceanographic conditions, both within the 1993 17 10,006 10,023 87 10,110 Maldives (Anderson, 1987 & 1993; Hafiz and Anderson, 1994) and within the wider western 1994 8 12,859 12,867 259 13,126 Indian Ocean (Marsac and Hallier, 1990; Marsac, Source: Ministry of Fisheries & Agriculture / EPCS 1992). It seems possible that oceanographic conditions during 1992-1994 were particularly favourable for yellowfin tuna in Maldivian waters. Annual average yellowfin CPUE decreased from a high of These points are discussed further below. 63 kg/day in 1981 to a low of about 27 kg/day in 1990. This decline has been noted before (Anderson, 1993; TRENDS IN CATCH PER UNIT EFFORT (CPUE) Anderson and Hafiz, 1991; Hafiz and Anderson, 1994;
The Maldivian yellowfin fishery is dominated by mechanized pole-and-line vessels. Annual average Figure 2.Percentage yellowfin catch by vessel type, 1970-1994 catches per unit effort (CPUE) for 1979-1994 for 100 these vessels are given in Table 2 and Figure 3. The best available measure of fishing effort, and 80 the one used here, is the number of fishing days. The problems associated with using fishing days as 60 a measure of pole-and-line fishing effort are well- known (e.g., Anderson, 1993; Hafiz and Anderson, 40 1994). They include problems of variations in bait Percen20tage of catch availability, seabird abundance, vessel interactions, and others. These difficulties mean that individual 0 annual estimates of Maldivian CPUE may not be too accurate, but the time series is believed to give 1970197119721973197419751976197719781979198019811982198319841985198619871988198919901991199219931994 a very useful picture of major trends. Sailing P/L Mechanized P/L Trolling
2 The Maldivian pole-and-line Table3. Indices of Maldivian yellowfin CPUE (kg/mech. P/L vessel day) for two seasonal fishery for yellowfin is highly yellowfin fisheries, 1970-1994. Data source: Ministry of Fisheries and Agriculture. seasonal. Peak catches are A: North East Monsoon. B: South West Monsoon made off the west coast of the Location: Kaafu, Malé town and Lhaviyani Location: Raa and Baa Atolls Maldives during the Months: Dec, Jan, Feb and March Months: June, July Aug and Sept. southwest monsoon (June to Year Catch (t) Effort (d) CPUE Year Catch (t) Effort(d) CPUE October), and off the east 1969-70 132 3,967 33 1970 827 6,345 130 coast during the northeast 1970-71 68 4,558 15 1971 376 4,976 76 monsoon (December to April) 1971-72 147 4,682 31 1972 1022 5,535 185 (Adam, 1993; Anderson, 1985 &1988; Hafiz and Anderson, 1972-73 430 6,227 69 1973 1814 6,671 272 1991; Rochepeau and Hafiz, 1973-74 327 6,555 50 1974 1294 5,555 233 1990). Particularly high 1974-75 776 5,674 137 1975 1644 12,393 133 catches are made off Raa and 1975-76 296 5,954 50 1976 1909 11,420 167 Baa Atolls during the 1976-77 688 5,787 119 1977 1358 9,336 145 southwest monsoon, and off 1977-78 418 6,255 67 1978 1131 9,079 125 Kaafu and Lhaviyani Atolls during the northeast monsoon. 1978-79 278 4,045 69 1979 1075 6,449 167 As simple indices of CPUE 1979-80 396 6,807 58 1980 1378 6,313 218 for these two seasonal 1980-81 539 7,037 77 1981 1160 5,047 230 fisheries, pole-and-line 1981-82 186 6,463 29 1982 1898 5,897 322 catches and effort have been 1982-83 517 7,947 65 1983 2175 6,706 324 compiled for the following two areas and time periods: 1983-84 1,234 12,373 100 1984 1527 8,805 173 1984-85 1,108 11,183 99 1985 1591 7,820 203 1. Southwest monsoon 1985-86 1,516 11,571 131 1986 1053 8,804 120 fishery: 1986-87 1,165 11,219 104 1987 2226 8,244 270 1987-88 984 11,800 83 1988 1062 7,513 141 • Raa and Baa Atolls, for 1988-89 571 12,360 46 1989 917 10,403 88 June, July, August and September. 1989-90 261 8,164 32 1990 550 12,483 44 1990-91 952 11,416 83 1991 778 9,597 81 2. Northeast monsoon 1991-92 892 10,705 83 1992 994 10,048 99 fishery: 1992-93 1,228 12,545 98 1993 1183 12,807 92 1993-94 1,818 12,544 145 1994 1438 13,115 110 • Lhaviyani and Kaafu Atolls and Malé town, for Note: To standardize sailing pole and line vessel effort to "number of mechanized pole and line vessels days" it was assumed that sailing vessels caught half the yellowfin caught by mechanized December January, vessels. For the years 1970-1978 sailing vessel effort was halved. For the years since 1979 sailing February and March. vessels effort was small and has been ignored. Also note that data for Dec. 1969 have not been included. The complete time series for both fisheries are presented in IPTP, 1992; Nishida, 1991). Possible reasons for this Table 3 and Figures4a and 4b. Note that fishing effort in decline are noted by IPTP (1992, p. 49). these time series has been standardized to “number of days fished by mechanized pole-and-line vessels”. This has been Since 1990 the annual average yellowfin CPUE has more done for years prior to 1979 by assuming that sailing than doubled, rising steadily from about 27 kg/day in 1990 vessels caught half the yellowfin that mechanized vessels to 59 kg/day in 1994. Part of this increase can be explained did, following Anderson (1985). by an increase in fishing power of the pole-and-line vessels, as noted above. Although the change in vessel Historically, the southwest monsoon fishery has always fishing power has not been quantified, it seems unlikely been more important than the northeast monsoon fishery. that it could account for more than about 10-20% of the The average CPUE for the southwest monsoon fishery increase in yellowfin CPUE over the last 5 years. The most during the 1970-1994 period was 165 kg of yellowfin per likely explanation for the remaining increase in Maldivian mechanized pole-and-line vessel day. For the northeast yellowfin catch and CPUE is changes in the oceanographic monsoon fishery the average CPUE was 81 kg/day. Both conditions in the Indian Ocean, discussed below. fisheries show similar recent trends in CPUE: a rapid decline from 1983/85-1990, and an increase since 1990. While there are a number of possible explanations for this
3 Table 4. Average lengths and size ranges (cm) of yellowfin tuna samples in Maldives during 1994. Sample Sample location Mean Mode Median Size range Smallest Largest Total in number 5-95% sample 1 H.Dh. Kulhudhufushi 46 48 47 33-57 25 82 11,730 2 R. Alifushi 46 47 30-60 20 84 10,063 3 K. Malé* 53 44 45 31-112 22 164 2,749 4 M. Maduvvari 44 47 47 33-51 29 154 2,269 5 Dh. Kudahuvadhoo 39 38 39 31-49 23 66 9,984 6 L. Maamendhoo 47 46 47 37-53 20 118 6,826 7 G. A.Villingili 43 43 44 32-55 17 87 12,283 8 G.Dh. Thinadhoo 47 47 47 35-58 20 98 18,563 TOTAL 74,467 *Malé sample includes catches from hand line and trolling pattern of changes, the most likely is believed to be (Euthynnus affinis), while the opposite pattern is seen in changes in oceanographic conditions, which are discussed skipjack, these trends are believed to be related in some below. way to medium-term variations in oceanographic conditions (Anderson, 1993; Hafiz and Anderson, 1994). Since 1990, CPUE in the southwest monsoon fishery has increased only slowly. In contrast the increase in CPUE in Variations associated with ENSO events. Maldivian the northeast monsoon fishery has been very rapid. During yellowfin CPUE increases during El Niño years (Anderson, 1970-1990 the southwest monsoon fishery CPUE was 1987 & 1993; Hafiz and Anderson, 1994; Rochepeau and greater than that of the northeast monsoon fishery in 20 out Hafiz, 1990). Note the elevated yellowfin CPUEs during of 21 years. During 1991-1994 the northeast monsoon the El Niño years of 1972-73 (Figures 1 and 4a), 1976 fishery CPUE was greater than that of the southwest (Figures 1 and 4b), 1982-83 (Figure 4b), 1987 (Figures 1, monsoon fishery in three out of four years. One possible 3, and 4b), and 1992-94 (Figures 1, 3, and 4a). In the explanation for this change is the increasing catch of western Indian Ocean, ENSO events are characterized by yellowfin tuna by other nations in the western Indian high sea-surface temperatures, low wind mixing, and Ocean, adversely affecting recruitment to the southwest strong vertical gradients in the thermocline. These monsoon fishery in the Maldives. conditions appear to promote yellowfin larval survival (Marsac and Hallier, 1990; Marsac, 1992) and hence presumably also recruitment to the Maldivian fishery. OCEANOGRAPHIC VARIATIONS AND YELLOWFIN CATCHES It appears that oceanographic conditions during 1992-1994 were particularly favourable for juvenile yellowfin tuna in As noted by Marsac (1992), a traditional approach to Maldivian waters. This is reflected in the high total catch population dynamics based on an assumption of (Figure 1) and national CPUE (Figure 3). However, the environmental stability is no longer tenable. In the relatively low CPUE for juvenile yellowfin off Raa and Maldives the abundance of juvenile yellowfin is known to Baa Atolls during the southwest monsoon is a cause for be affected by variations in oceanographic conditions of at concern. There is clearly a need for further study of both least three types: the effects of oceanographic variability on yellowfin tuna distribution and abundance, and of fisheries interactions, Seasonal variations. As mentioned above, the fishery for within the Indian Ocean. juvenile yellowfin tunas in the Maldives is highly seasonal. In fact there are essentially two fisheries, one off the west coast during the southwest monsoon and one off the east THE FISHERY FOR LARGE YELLOWFIN coast during the northeast monsoon (Adam, 1993; Anderson, 1985 & 1988; Anderson and Hafiz, 1991; The great majority of yellowfin landed in the Maldives are Rochepeau and Hafiz, 1990). In both seasons the juvenile surface-swimming juveniles, caught by live-bait pole and yellowfin are strongly associated with drifting objects. line. A traditional trolling and handlining fishery for large yellowfin also exists in the Maldives. The yellowfin caught Medium-term variations. As noted above, there have in this fishery are generally subadults and adults of more been medium-term trends in yellowfin CPUE in recent than 70cm FL. Large yellowfin are also caught years. Since the same pattern of variation in CPUE is seen occasionally by pole and line, using double poles. There in Maldivian frigate tuna (Auxis thazard) and kawakawa
4 In addition to the “inshore” fishery for large yellowfin, Figure 3. Mechanized pole and line CPUE for yellowfin tuna, 1979-1994, for the entire Maldives (kg/day) some tuna longlining has been and is being carried out in 70 the outer waters of the Maldivian EEZ (i.e. 75-200 miles 60 offshore). Further details are given in Anderson, Hafiz 50 and Adam (1995).
40
30 UE (kg/day)
P SIZE DISTRIBUTION OF YELLOWFIN C 20 CATCHES 10
0 In 1993 Maldives initiated a regional tuna length-
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 frequency sampling program. Active pole-and-line skippers were employed to measure their tuna catches at
Figure 4a. CPUE of yellowfin tuna for Lhaviyani, and Kaafu Atolls and Malé town, NE monsoon season seven islands, representing all regions of the Maldives. (kg/mech. P/L day) In addition, sampling at Malé market, which was 350 initiated in 1983, has continued. 1994 data are 300
250 summarized in Figure 5 and Table 4. Among over
200 74,000 yellowfin measured in 1994, the commonest 150 sizes in the pole-and-line fishery were between 38 and UE (kg/day)
P 100 C 50 cm FL. The great majority of the yellowfin sampled 50 fell within the 30-60cm FL size range. There was no 0 obvious trend in yellowfin size between regions. Yellowfin sampled at one location (Dh. Kudahuvadhoo) were on average smaller than at other locations. The median size of yellowfin sampled there was 39 cm FL, compared with 44-47 cm FL at the other seven sampling Figure 4b . CPUE of yellowfin tuna for Raa and Baa Atolls, SW monsoon season (kg/mech. P/L day) locations. As Dh. Kudahuvadhoo is a central location, 350 this observation is difficult to explain; returns for 1995 300 are currently being compiled and may shed some light on 250 this apparent anomaly. 200
150 The regional length-frequency sampling programme UE (kg/day) P C 100 concentrates on pole-and-line catches, with the exception 50 of Malé market sampling which covers all gears. As a 0 result, large yellowfin catches are not particularly well represented. Some sampling of large yellowfin catches are a number of well-established local fisheries for large has been carried out in Haa Alifu, Raa and Baa Atolls in yellowfin, including ones off: the north and at Fuvah Mulaku in the south. The combined length-frequency distribution of handline catches of large • Haa Alifu Atoll, in the far north, during January-April. yellowfin from these two locations is presented in Figure 6. • Malé, in the centre of the country, during March- Sampling at Malé does not separate pole-and-line and September. handline catches, so Malé data have been excluded from Figure 6. In both Figure 5c (Malé) and Figure 6 there is a • Fuvah Mulaku and Addu Atoll, in the far south, during clear mode at 82-84cm FL. The 108cm FL mode in Figure April and November. 6 is mainly due to fish from Fuvah Mulaku. The Fuvah Mulaku fishery has been briefly described by Anderson, Adam and Waheed (1993). However, the overall OTHER BIOLOGICAL INFORMATION seasonal distribution of large yellowfin tunas within Maldivian waters has not been well documented. A survey Growth. Because the Maldivian yellowfin tuna fishery has therefore been initiated of all fishing islands in the mainly targets juveniles, it has not been possible to develop country, to obtain information from experienced fishermen growth models for the full size range of yellowfin tuna on the seasonal occurrence of large yellowfin. At present found in Maldivian waters. Two studies so far have there is considerable business interest in large yellowfin concentrated on growth rates of juveniles. From analysis of within the Maldives: exports of chilled fish to the Japanese length-frequency data, Anderson (1988) estimated a linear sashimi market and of tuna loins to Europe have recently growth rate of 2.9±0.4 cm/mo between 30-70cm FL started. (although growth at half this rate could not be discounted). From tagging data, Yesaki and Waheed (1992) estimated an average growth rate of 2.4 cm/mo at 70cm FL.
5 fish” (Montaudouin, FigFiugurree 6.5 .L eLengthngth freque ncfrequencyy distribution ofdistribution "Large Yellowfin Tofuna" ye calluoghtw infi tnhe tMalundaiv efsr byo handlm thinee andM aldives, 1994. Hallier and Hassani, 1990; troll . Hallier, 1991). Maldeniya and Joseph (1988) 70 1800 H. Dh. Kulhudhufushi, North East Maldives 1800 Dh. Kudahuvadhoo , Central West Maldives 1600 1600 demonstrated a northward
) 1400 s
) 1400 60 Total: 11730 s 1200 Total: 516 Total: 9984 1200 movement of yellowfin umber
n 1000
number 1000 50 800 along the west coast of Sri 800 600
equency ( 600
equency ( Lanka, mainly on the basis Fr 400
40 Fr 400 200 200 of changes in relative 0 0
30 18 28 38 48 58 68 78 88 98 108 118 128 138 148 158 168 18 28 38 48 58 68 78 88 98 abundance of 60-80cm FL Fork length (cm) 108 118 128 138 148 158 168 Fork length (cm) 20 fish. Frequency (numbers)
1000 R. Alifushi, North West Maldives 1400 L. Maamendhoo , South Maldives 10 900 1200 Regarding adult yellowfin, 800 Total: 6826 )
) Total: 10,063 s
s 1000 0 700 Morita and Koto (1971)
40 60048 56 64 72 80 88 96 800 104 112 120 128 136 144 152 160 number number 500 Fork Length (cm) suggested that there is a 600 400 equency ( equency ( movement of fish from the 300 400 Fr Fr 200 200 equatorial western Indian 100 0 0 Ocean, through the 18 28 38 48 58 68 78 88 98 18 28 38 48 58 68 78 88 98 108 118 128 138 148 158 168 108 118 128 138 148 158 168 Fork length (cm) Fork length (cm) southern Maldives and up past Sri Lanka into the Bay 450 Male' Market (Kaafu Atoll ), Central Maldives 2500 G.A.. Villingili, South East Maldives 400 of Bengal every year 2000 ) )
350 Total: 2749 s s Total: 12283 300 between October and 1500 number
number 250 200 March. The seasonal 1000 150 equency ( equency ( fishery for large yellowfin Fr Fr 100 500 50 at Fuvah Mulaku every 0 0 18 28 38 48 58 68 78 88 98 18 30 40 50 60 70 80 90 108 118 128 138 148 158 168 100 110 120 130 140 150 160 170 November may be Fork length (cm) Fork length (cm) targeting these fish 500 M. Madduvari, Central East Maldives 3000 G. Dh. Thinadhoo , South West Maldives 450 (Anderson, Shiham and ) 400 ) 2500 s s 350 Total: 2269 Total: 18563 2000 Waheed, 1993). Some 300 number number 250 1500 tagging of large yellowfin 200 150 1000 equency ( equency ( at Fuvah Mulaku has been Fr 100 Fr 500 50 0 0 carried out (Anderson, 18 28 38 48 58 68 78 88 98 18 28 38 48 58 68 78 88 98 108 118 128 138 148 158 168 108 118 128 138 148 158 168 Fork length (cm) Fork length (cm) Adam and Waheed, 1995), but no overseas recoveries have been received to date.
Stock relationships. Migration. Anderson (1988) proposed a model of juvenile Nishida (1992) proposed that there are two major stocks of yellowfin tuna migration in the central Indian Ocean in yellowfin tunas in the Indian Ocean: a western and an which a broad band of young fish in the equatorial waters eastern stock, with an area of overlap between about 70o - moves east and west in phase with the seasonally changing 90oE. If this is the case, it is possible that the juvenile monsoon currents. Anderson (1988) and Adam (1992) yellowfin caught off the west coast of the Maldives during suggest that intermediate-size fish migrate northwards from the southwest monsoon could come from the western stock, the Maldives into the north of the Arabian Sea. Length- while those caught off the east coast of the Maldives during frequency data from pole-and-line catches presented here the northeast monsoon could come from the eastern stock. show no evidence of increase in size with latitude within A scatter plot of southwest monsoon fishery CPUE against the Maldives. From a tagging study of juvenile yellowfin northeast monsoon fishery CPUE shows no obvious tuna in Maldives, Yesaki and Waheed (1992) confirmed correlation. The same applied when southwest monsoon the east-west movement in phase with the monsoons, but fishery CPUE was plotted against northeast monsoon did not present evidence for a net northward movement. It fishery CPUE for previous and following years. This may be therefore that if there is a northward migration it finding would tend to support the two-stock hypothesis. If does not start until the yellowfin have grow to a size that is the case, the similarity in CPUE trends over the greater than that at which they are normally taken by pole 1970-94 period for the two fisheries, noted above, might and line (i.e. greater than 60cm FL). For western Indian still be attributed to large-scale changes in oceanographic Ocean yellowfin, a change in body proportions determined conditions affecting local abundance or catchability in by detailed analyses of length-weight relationships has Maldivian waters of juvenile yellowfin tunas from different been noted at about 64-68cm FL, and this has been sources. interpreted as reflecting a “turning point in the life of this
6 Length-weight relationship. Anderson et al. (1995) have W = 0.00002863 FL 2.897 ( N = 875; r = 0.990 ) calculated the following length-weight relationship (cm-kg) for Maldivian yellowfin tuna landings, within the size range of 25-145cm FL:
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